Shielded spark plug



remnant. 6, 1945 zmias'al UNITED VSTATES 'PATENT oFFicn SHIELDED SPARK PLUG Earl 0. Sctterblade, Glen Rock, N. I., assignor to Wright Aeronautical Corporation, a corporation oi' New York e e 9 gaims This invention relates to spark plugs or inter.- nal combustion engines and in particular relates to improvements in electrically shie ded sparkv plugs suitable for use in aircraft engines.

One of the problems in connection with air- .craft engine spark plugs is the provision off a suiciently long flashover gap within the spark plug and between the center electrode connection and a groundedelement to prevent break/ down at high altitude'. It is an object of the present invention to provide a spark plug which will meet such operating conditions and further, to provide a labyrinth flashover path which is e' Application october 1o, 1941, seriamo. 414,445

(Cl. 12S-1.69)

not likely to break down under anyoperating l condition.v A further object of the invention is to provide a resistor, integral with the spark plug, to damp'surges of energy across the plug gap released from the condenser formed by the radio shielding which embraces the spark plug and the ignition system. Another object of the invention is to provide a spark plug connector assembly which may readily be filled with a semi-liquid insulating compound after the assembly has been completed, to assure proper mechanical protection and insulation ofthe spark plug assembly. Still another object off the invention is to provide an electrode arrangement whereby the sparking points will not tend to separate at high temperature operating conditions. A further object is to provide a spark plug electrode arrangement so arranged as to stay hot enough during engine -idling operation to give good engine idling characteristics and yet to run suiliciently cool under normal power output to prevent excessive erosion o -f the sparking points. The `details of thel invention are the ceramic and the electrode I1. The top part f outlined in the annexed description which may be read in connection with the drawing, in which: Fig. 1 is a longitudinal section through a spark plug according to the invention; and

' Fig. 2 is a view on the line 2--2 of Fig. 1. The spark plug proper comprises a. shell lll of steel or other hard material having `screw threads il engaging the usual'screw threads in a spark plug opening in' an engine, the shell ID having a flange I2 for applicationof a wrench. A plurality of outer electrodes I4 are suitably secured to the bottom of the shell lll and each comprises a strip or wire whose ends meet the shell I0 as at l5 and whose central portion forms a loop at I6 in clearance relation to the center electrode l1. Alternately, the several xed electrodes may be formed as a unit stamping secured at its' periphery to the shell l0. The loops I6 are bent upwardly as shown at i8 in Fig. 1,

' a portion of each loop being in nearly parallel relationship with the side faces of the conical center electrode I1. The normal disposition for outer electrodes is to have electrode ends bent vlll as by brazing, pressing, or the like and within this sleeve a refractory ceramic insulator 2l is molded. When the insulator 2l is molded, the centerelectrode I1 is imbedded therein and also, a carbon resistor 23 is imbedded therein, the electrode I1 being in intimate contact with the resistor 23 -at 24. The electrode I1 is of downwardly pointed conical form and comprises a copper core 25 embraced by a shell 26 of plati num alloy or other suitable metal adapted to resist high temperature and erosion. 'I'he resistor 23 is also ofconical form to provide a gas seal against any possible leakage between or the resistor 23 is capped with a resilient metallic washer 28 suitably formed to receive a wire terminal 29 of a high tension conductor 30.

Above the resistor 23, the insulator 2lv is formed with a central well 3| dened in part by a cylindrical skirt 32 formed integral with the insulator 2l. Also integral with the insulator 2l is an outer cylindrical skirt 33 in contact with y the sleeve 20, the two skirts 32 and 33 .dening therebetween an annular well.

The high tension conductor is embraced by metallic shielding-35 secured to a union fitting 36 which contacts a metallic ,elbow 31 to which it is secured by a union nut 38. The fitting 36 contains a rubber gasket 40'w-hich seals the conductor 30 to the fitting against leakage and as the conductor 30 passes throughthev elbow 31,

there is a space between these two parts which is filled by a semiliquid insulating compound shown at 4|, `through a fitting 42 attached to the elbow, said fitting having a small opening 42 through which the compound may be introduced by a pressure gun of suitable type. vThe .bottom of the elbow 31 is bonded to a'conical shell 43 whose lower edge rests upon the top of the sleeve 20, the shell and sleeve being secured to one another by a union nut 44 engaging threads on the sleeve, said nut being provided with holes l for application of a spanneiwrench thereto.

Within the shell 4l and the upper part of the sleeve 2l is an insulating cap 41 sealed to the shell by a rubber gasket 4B, Through said cap the conductor Il passes, the cap comprising a central stem 49 extending down into the well Il, and also comprising an annular substantially cylindrical skirt which passes into the annular space between the skirts 32 and 33 of the'insulator 2|. 'I'he cap insulator `and the insulator 2l together define a circuitous fiashover path of substantially 8 form whose length, obviously, is much greater than would obtain if the labyrinth established by the interiltting insulator skirts were absent. It will be further noted that the limbs of the fiashover path are parallel to one another which disposition seems to afford further restriction to iiashover. That is, the effective length of the flashover gap shown is greater than its actual length, from the standpoint of breakdown potential, so that 'a higher breakdown potential is 'necessary for the gap shown than would be predicted from the actual gap length.

The lower end of the conductor 30 is secured in the cap extension 49 by a metallic wedge 52, which wedge may be soldered or otherwise fastened to the protruding end of the conductor to form the contactor 29 which engages the metal washer 28. The latterincidentally, should be of slightly resilient construction to assure positive contact with the contactor 29.

In assembling the conductor and elbow unit, the insulation of the conductor 30 is first coated with sealing compoundand it is inserted into the bore of the insulating cap lil. The wedge 52 is applied and the elbow 3l and its fittings are made secure, then sea-ling compound is injected through the tting l2, completing the wire terminal which may be applied subsequently to the spark plug base unit.

It is believed that a plug constructed in accordance with the teachings above will have much greater life than conventional spark plugs and if, as anticipated, the plug is capable of operating properly for a period of 600 hours or more, it will be unnecessary to remove the spark plugs from an engine lbetween maior overhauls. If this hope is realized, engine construction may be simplified to the extent that the usual hardened spark plug bushings which are shrunk into cylinder heads, may be eliminated. The infrequent withdrawal of the spark plugs will allow of spark plug assembly directly into the aluminum cylinder head without need for said cylinder head bushings and thereby affording improved heat transfer from the plug to the cylinder.

aaea'eeo v ceramic insulator having lannular undulations complementary in form to those olf the other insulator, means to secure said insulators in Juxelectrode, secured in the other end of said pas- While I have described my invention in detail v in its present preferred embodiment, it will be obvious to those skilled in the art, afterunderstanding my invention, that various changes and modlcations may be mad therein without departing from the spirit ,or scope thereof. I aim sulator having a plurality of substantially cylin drical annular undulations therein between the conductor and shell; a top member comprising a sage, and contactor means secured to said shell and engaging said resistor, said insulator having an integral annular skirt projecting from the resistor end thereof, and said contactor means comprising a plurality of integral concentric annular refractory skirts between two oi' which said insulator skirt is assembled, whereby an elongated fiashover path is defined' between said. shell and resistor, part of said flasl'iover path comprising spaced parallel portions.

3.. In a spark plug, a refractory cylinder comprising a core, a shielding sleeve embracing same, an attachment shell secured to said sleeve toward the lower end of said cylinder, the lower end of said cylinder being recessed and having a central electrode imbedded therein, a carbonaceous resistor engaging said electrode and imbedded in said cylinder above the electrode, said cylinderhaving formed therein a central well and a concentric annular well around'the central well; a

contactor assembly comprising a centrally bored insulator having a conductor passing there` through for engagement with said resistor, a portion of said insulator lying within each well of said refractory cylinder `to define a circuitous flashover path from said conductor to said shielding, and a shielding cap embracing said conductor and a portion of said insulator, secured to said shielding sleeve.

4. In a shielded spark plug for an engine, e.

base portion secured to the engine having a shell,

a refractory insulator and a pair of electrodes, one electrode secured to the shell and the other imbedded in the insulator, said insulator comprising at its end opposite to the electrodes a plurality of integral annular substantially cylindrical concentric skirts defining open ended annular spaces therebetween; and a cap portion having a metallic casing secured to said shell, an insulating member having a plurality oi' integral annular substantially cylindrical Iconcentric skirts lying within said annular spaces of said refractory insulator, and a conductor passing through said cap portion and having electrical contact with said imbedded electrode.

5. In a shielded spark plug for an engine,4 a base portion secured to the engine having a shell, a refractory insulator and a pair of electrodes, one electrode secured `to the shell and the other imbedded in the insulator, said insulator comprising `at its end opposite to the electrodes a plurality of intgral annular substantially cylindrical concentric skirts defining open ended annular spaces therebetween; and a cap portion having a metallic casing secured to said shell, an insulating member having a plurality of integral annular substantially cylindrical concentric skirts lying within said annular spaces of said refractory insulator, and a conductor passing through said cap portion and having electrical contact with said imbedded electrode, said imbedded electrode comprising a metallic sparking point and a carbonaceous resistor the latter being wholly conned within the refractory ins'ulator except where it is contacted by said sparksecured to said conduit having a restricted hole therethrough through which a viscous insulating compound may be introduced into the space between said conduit and insulating material to seal said space, and gasket means at each end of said space to ,prevent leakage of said compound therefrom.

8. In a'shielded spark plug having a pair of electrodes, a base portion comprising an insulator member having a passage therethrough, a metallic shell disposed about said insulator member and connected to one of said electrodes, electric conductor means withinl said passage and including the other 0i said electrodes, said insulator member having a plurality of substantially cylindrica1 annular Iribs projecting therefrom, a cap portion comprising an insulator member havingY a plurality of substantially cylindrical annular ribs adapted to be received between the annular ribs projecting from said base portion, a metallic shell disposed ^about said cap portion connected to said rst mentioned shell, and conductor means within said cap portion having electric contact with said rst mentioned conductor means.

9. In a shielded spark plug, a spark plugl'cap member having an insulator portion and a metallic shell, said shell having a metallic conduit portion extending therefrom, a conductor member extending through said conduit and insulator portions,4 and .a viscous sealing compound disposed between said conductor member and conduit and between said shell and insulator portieri. 

